Add new purchasing options to shops

[rekterakathom]

Add some new buttons to the purchasing UI, like 'buy 5, buy 10, buy 50, buy max'.

Currently having a macro to purchase stuff is basically required when making money by selling things between towns, and even with a macro it is rather slow. By the time you are moving ~500 items or more each trip, most of the time gets spent in clicking (or macroing) the buy button.

[Dzemoglowy]

I have a question I'm thinking what skills it's possible to do it? Basics of C++? I found some simillar code in warehouse interface. Maybe it's possible to use it in store interface.